1. Field of the Invention
The present invention relates to a magnetic disk medium, and particularly to a magnetic disk medium of a magnetic disk unit which is an external storage apparatus of a computer.
2. Description of the Related Art
In a conventional magnetic disk unit, a magnetic disk medium is used as a storage medium. A magnetic head in a stationary state contacts the magnetic disk medium when the disk unit is power-off, slides on the magnetic disk medium as the magnetic disk medium rotates when starting up, and then floats up by an air flow generated between the magnetic head and the magnetic medium and performs a read/write operation in the floating state.
This is called a contact start stop system (CSS system) and is adopted in many kinds of magnetic disk units.
Such a magnetic disk medium comprises an underlayer 2, a magnetic layer 3, a protective layer 4, and a lubricant 5 each made of a thin film provided one after another on a non-magnetic substrate 1, as shown in FIG. 4. The protective layer 4 is formed for the purpose of protecting the magnetic layer 3 from corrosion caused by an external environment and from wear due to sliding contact with a magnetic head in a CSS system, and the like.
Although various kinds of materials such as amorphous carbon, SiO.sub.2, SiC, ZrO.sub.2, and the like have been proposed as a material of the protective layer 4, the most popular material is a material belonging to an amorphous carbon system, which is widely used in a state where it is improved in mechanical strength by adding hydrogen to it.
In order to increase recording density, it is important to reduce a magnetic spacing between a magnetic head and a magnetic disk medium in read and write operations. The magnetic spacing is the distance between a magnetic layer of a magnetic disk medium and a recording/readout element of a magnetic head. Since the magnetic spacing includes also the thickness of the protective layer 4, it is desired to make the protective layer 4 thinner. However, when the protective layer 4 is made thinner, there results degradations of the magnetic layer 3 in corrosion resistance and wear durability.
Therefore, the present inventor reviewed how thin the protective layer can be made, for a protective layer made of conventional hydrogenated carbon. The result of the review was reported in Kuratomi et al., "An influence of making hydrogenated carbon protective layer thinner on CSS durability" Japan Society of Tribologists, the Proceeding of Tribology Conference, Tokyo, May, 1995, pages 85-88.
It was made clear that, when the thickness of protective layer decreased from 10 nm to 5 nm, wear characteristics of the protective layer change substantially and it cannot keep sufficient wear durability against sliding contact with the magnetic head during start-up and stop period in contact start stop systems.
As a result, it was found, wear proceeded rapidly and CSS (Contact Start Stop) durability of a magnetic disk medium degraded rapidly.
It was also demonstrated that substantial degradation of wear durability at 5 nm of protective layer thickness was caused by an internal stress distribution of the protective layer in the direction of layer thickness, and a degradation mechanism by internal stress was proposed.
That is, an internal stress of thin film is generated by internal strain such as a deviation of mutual location of atoms or microscopic clusters.
When amorphous carbon system material is formed on a magnetic layer made of magnetic metal alloy, the protective layer is heavily influenced by interaction with the magnetic layer, such as by the difference of thermal expansion coefficient or the like and the effect of interface mismatching. Therefore, a protective layer directly overlying the magnetic layer yields large strain in the "in-plane" direction perpendicular to layer thickness (hereafter in-plane).
Accordingly, as the protective layer thickness increases and a distance of the read-out head from the interface of magnetic layer becomes larger, the strain is relaxed and the amount of in-plane strain steadily decreases toward a value of about "0" as shown in FIG. 1.
As a result, in-plane internal stress is large in a domain adjacent to magnetic layer due to the in-plane strain, while in-plane internal stress is relieved at a longer distance than a certain layer thickness, because in-plane strain is relaxed. According to the measurement result of stress vs. the layer thickness, in the conventional protective layer, in-plane compression internal stress is large within 5 nm of the interface of the magnetic layer, while in an area at a distance of more than 10 nm from the interface of the magnetic layer, in-plane compression internal stress is almost non-existent compared with the area within 5 nm of the interface of magnetic layer.
A thickness of about 20 nm is adopted as the thickness of the protective layer 4 of a conventional magnetic disk medium, and this thickness is enough to achieve good wear durability; however, since a thickness of the protective layer of 20 nm causes magnetic spacing to be too big, it is needed to make the protective layer thinner in order to increase recording density.
Since a conventional protective layer 4 degrades remarkably in its own wear durability, when the layer thickness is reduced to 10 nm or less, it cannot keep a sufficient wear durability against a magnetic head when starting and stopping a disk unit of CSS system and is liable to damage the magnetic layer 3.